The present invention relates to a method and furnace for the heat treatment of a scale-covered steel product.
It is generally known that most metals tend to be incrusted with scales under the influence of heat and ambient atmosphere in refinement or any other processing. The scales so formed are generally membranous outgrowths of the skin and are generally called "mill scales" when formed on hot-rolled metallic products. It is also well known that, under the influence of rain and/or ambient atmosphere, rust tends to develop on the hot-rolled metallic product where some scales have been peeled off because of rough handling of the product.
In view of the above, it is a conventional practice to heat a hot-rolled metallic product for improving the workability and also to remove the scales from the hot-rolled metallic product, prior to the third process being worked thereon. According to the prior art, as shown in FIG. 1 of the accompanying drawings, a steel product, for example, a hoop, wire or rod, is pickled with either sulfuric acid or hydrochloric acid to remove mill scales therefrom during a first pickling step 1. The steel product so treated is subsequently heat-treated in a second step 2 under a reducing atmosphere. During the second heat treatment step 2, the steel product is heated to an appropriate temperature of 650.degree. C. or higher depending on the type of the product, for a predetermined period of time and is then slowly cooled to a temperature of 650.degree. to 500.degree. C., the steel product being thereafter allowed to stand in the atmosphere after it has been cooled to the temperature range, as shown in FIG. 2 of the accompanying drawings which illustrates a heat cycle effected on the product during the heat treatment step 2.
The steel product so heat-treated tends to be incrusted with scales in contact with oxidizing elements in the air while it is allowed to stand subsequent to the heat treatment step 2. Therefore, the steel product with scales thereon is again pickled during the second pickling step 3 to remove the scales completely, i.e., to remove the scales including not only those formed subsequent to the heat treatment step 2, but also those left unremoved during the first pickling step 1.
Since the prior art method described above requires the pickling operation to be performed two times, i.e., before and after the heat treatment, not only is it time-consuming and uneconomical, but also the acid used during each pickling operation may dissolve the work being processed to such an extent as to result in the reduced quality of the steel product.